Numerous systems for the freeform fabrication of three dimensional objects under computer control have been proposed. This field has become known by the general terms "rapid prototyping" and "desktop manufacturing" over the last several years.
While existing or proposed three dimensional freeform fabrication systems differ from each other in the materials used to build objects in the specific process, the form or state of these materials, and the particulars of the mechanics to form objects and the properties of the resulting objects themselves, almost all of the methods are based upon the layerwise superposition and bonding of materials to form the object. The numerical information required to control a freeform fabrication apparatus and thus to form and bond together each such layer of an object is commonly obtained by performing additional mathematical processing upon the data file which defines the desired object from a three dimensional computer aided design (CAD) system. These additional mathematical steps define the layers of the object as required to perform a freeform fabrication process. It is also possible to use as a starting point a physical object, digitize the spatial coordinates of the object and in a similar fashion perform additional mathematical processing on this data to define layers of the object for a freeform fabrication process. Thus, a second object can be generated that may be different in scale, materials or design from the first.
The uses for three dimensional freeform fabrication systems include, but are not limited to: the rapid fabrication of prototype parts for use as engineering or design models, both for functional and visual design verification; the rapid fabrication of investment and other types of casting patterns; fabrication of unique objects to be used as casting patterns or even functional objects, as for example from CAT scan or NMR data for prosthetic and other medical uses; the fabrication of objects of geometries that would be difficult or impossible to realize using typical subtractive machining processes such as milling or turning on a lathe.
Such systems in addition possess the virtue of allowing rapid realization of a three dimensional object from data supplied by a computer aided design (CAD) system without the preparation of intervening tooling and thereby lower the time for the overall design cycle in many cases from months to a few days or hours. This results in great savings in tooling costs, especially in cases where several iterations may be required to be performed until an acceptable final design or tool is realized, and also results in a highly-desirable, more timely introduction of the so-designed products to market.
Among the processes that have been proposed are those which use layerwise hardening of a photopolymer as the object medium such as stereolithography, U.S. Pat. No. 4,575,330, solid ground curing, U.S. Pat. No. 4,961,154, and design controlled automatic fabrication, U.S. Pat. No. 4,752,498, and others. Another group of proposed technologies is based on layerwise material deposition and includes ballistic particle manufacturing, U.S. Pat. No. 4,665,492, fused deposition modeling, U.S. Pat. No. 5,121,329, inkjet methods, U.S. Pat. No. 5,059,266, weld metal deposition, U.S. Pat. No. 5,207,371, masked plasma spray, U.S. Pat. No. 5,126,529, and others. The object materials in these techniques include melt extruded plastics and waxes, photopolymers, ballistically jetted waxes and metals, and other materials. Still other methods have been proposed based upon the bonding of powders. Selective laser sintering, U.S. Pat. No. 4,863,538, uses the energy of a laser to layerwise bond particles of plastic, wax, or metal together until the desired object is formed. Three dimensional printing, U.S. Pat. No. 5,204,055, is similar, but replaces the laser with a high speed inkjet system which ejects bonding material layerwise into a bed of ceramic powder. The object so formed may subsequently be sintered and may be used to produce a metal article by means of an investment casting-like process without the requirement for an intervening wax or other material pattern.
Additional three dimensional freeform fabrication methods are based on the cutting out of cross sections of the desired object from sheet or web fed material and subsequent lamination of these cross sections to form the object. The most commercially successful of these techniques to date is laminated object manufacturing, U.S. Pat. No. 4,752,352. Paper, plastic films of various kinds and metal foils may be used in this process to form the desired object.
While there are, as described above, many proposed methods for three dimensional freeform fabrication, and indeed some of these have reached a significant level of commercialization, there are many disadvantages to the existing and proposed methods. Examination of these methods and their characteristics, advantages and disadvantages, permits the desirable properties of an improved freeform three dimensional object fabrication method to be listed:
a. It is desirable that an improved method of freeform fabrication be a slice-based technology. This allows the rate of construction of the object to be independent of the geometry of the object and shortens fabrication time by eliminating the need for calculating and individually positioning vectors as is required for stereolithography or fused deposition modeling. It would be a further advantage if the layers did not require the fabrication of an intervening mask as required by solid ground curing or design controlled automatic fabrication. PA1 b. A solid support material is desirable in an improved method of freeform fabrication in order to produce desired objects with the most generalized geometric capability, to minimize built in stress and to improve accuracy by preventing wandering and swelling of the fabricated object as occurs with some methods that use liquid photopolymers. A solid support also dispenses with the need to design a support structure for overhanging or other geometrically awkward volumes of the desired object as is required in stereolithography or fused deposition modeling. PA1 c. It is desirable in an improved method of freeform fabrication that the support material be subjected to the same physical processes as the build or object material in order to result in minimum differences in physical properties between the support and the object materials. The reasons are similar to those set forth above. Note that in the case of selective laser sintering, while a solid support structure is provided by the powder which remains unsintered, the difference in density between this unsintered support and the sintered object powders can lead to inaccuracies in object geometry. PA1 d. An improved method of freeform fabrication should be capable of providing high resolution and accuracy. High resolution will result in better surface finish. PA1 e. High speed operation should be possible with an improved method of freeform fabrication. It is very desirable to be able to build objects much more quickly than previously known methods are capable. PA1 f. Object materials used in an improved method of freeform fabrication should be safe, non-toxic and inexpensive. Unlike the situation with some existing methods, it is highly desirable to be able to build objects in materials which are suitable for the actual application envisioned for the object. It is further desirable that there results no emission of smoke or vapors requiring venting as with some methods such as laminated object manufacturing which uses a carbon dioxide laser for material cutting. Many photopolymers used in present methods can irritate the skin or respiratory tract and are suspected carcinogens. It is thus desirable to avoid the use of these materials. PA1 g. An improved method of freeform fabrication should utilize existing technology and not require expensive or exotic components. Many present methods such as stereolithography require expensive and/or limited-life lasers, expensive laser beam positioning and modulating means or other costly components. The use of less expensive components will result in lower prices for the equipment and wider adoption. PA1 h. An improved method of freeform fabrication should evolve from technology with a record of reliability. PA1 i. The post-processing operations of an improved method of freeform fabrication, if any, should be clean and simple. Some present methods such as stereolithography require post-curing of fabricated objects in an oven or UV light box after removal of excess material by solvent-bathing and manual wiping. Other methods offer much easier post-processing such as a warm water rinsing, or brushing off of excess powder, and there are some methods where no post-processing is required at all. These cleaner techniques of post processing are highly desirable. PA1 j. An improved method of freeform fabrication should be adaptable to many market segments: for example, low-cost, high-speed, multi-material, etc. The cost of components in many technologies, precludes low-cost versions of such equipment from being developed, and the nature of many of the processes is such that they may not be adaptable to a wide array of such markets. PA1 k. The size and other mechanical characteristics of the machinery used in an improved method of freeform fabrication should be appropriate for an office or laboratory environment. Some present systems such as solid ground curing weigh several tons and occupy a complete room.
Thus, it can be seen that in spite of the existence of numerous proposed and current methods for the fabrication of freeform three dimensional objects, there still exists a need for a method which combines the desirable characteristics described above.
U.S. Pat. No. 5,088,047 teaches a prior art method of freeform fabrication based on electrophotography that answers the requirements described above. See, for example, Schein, L. B., "Electrophotography and Development Physics," Springer-Verlag, Berlin 1988, for a general review of the field. In this patent, a powder image representing a thin laminar cross section of a desired object is formed on a conductive drum having its exterior surface coated with a photoconductor in the usual manner of electrophotography. A uniform charge is placed on the drum by a corona or other discharge element and this charge is subsequently dissipated imagewise by an exposure element. The exposure element according to this patent may be any of several known optical devices including, but not limited to, a scanned laser, light emitting diode arrays and liquid crystal gated linear light sources. The latent image so formed corresponds to a single cross section of a desired object.
This latent image subsequently rotates through one or more developing stations where various object building substances in powder form are deposited by electrostatic attraction to the latent image on the drum. The drum rotates in synchronism with and in close proximity to or in contact with a dielectric transfer belt. Charge of a correct sign and magnitude deposited on the belt by a second corona or other discharge element electrostatically attracts the deposited object building powders imagewise to the belt. Subsequently, the lamina are moved to a build area where the object is assembled from the series of lamina.
While all the desirable criteria listed above are met by the prior art there still exist several shortcomings when the prior art is used for three dimensional freeform fabrication:
First, owing to the photoconductive nature of the processes involved in electrophotography, all machine operations must be conducted in the dark and a machine which utilizes this process must be made light-tight. In addition to the expense of providing the additional machine elements to accomplish this end, it is highly desirable that the progression of the fabrication of objects be viewable by an operator. Since in many cases such an apparatus will be constructing never before made objects, it is desirable to be able to continuously monitor the progress and to be able to stop or modify operation in the event of error.
Second, the development of object building powders to be used in an electrophotographically based three dimensional freeform fabrication process is complicated by the requirements for simultaneous compatibility of a wide range of properties for each such material: Object building powders must be compatible for use with the chosen photoconductor material, and must simultaneously possess compatible and desirable dielectric, thermal, chemical, fusing and mechanical properties. A means that decreases the number of properties that must be required simultaneously of object building powders is highly desirable and will result in both a greater number and range of useful materials, and in fewer restrictions on each material thus used.
Third, a method of three dimensional freeform fabrication which is based upon electrophotography is capable of making very thin object layers and providing very high resolution along the thickness or object building axis. Indeed, electrophotographic processes inherently produce thin layers. In a freeform object fabrication application, this results in the requirement for a very large number of machine operations. For example, a 10 inch high object made from 0.00033 inch thick layers results in a total of 30,000 layers. Photoconductors experience both mechanical wear as well as degradation in charge trapping characteristics with such prolonged use and thus a method which provides a large number of cycles of operation of the latent imaging member is highly desirable.
While there are available photoconductors such as amorphous silicon that are guaranteed by manufacturers for 500,000 operations, and have been tested to 1,000,000, this latter figure would only represent 330 height inches of object using the example above. This result can also be thought of as only 33 ten inch high objects. Use of such photoconductors may also require significant additional limitations on object building powders. For example, Kyocera Corp. of Japan has recently introduced the use of amorphous silicon photoconductors on a commercial basis in low cost laser printers. The toner used with this latent imaging member requires the addition of abrasive particles to keep the photoconductive drum polished and thereby achieve a long service life.
While techniques known in the art as have been applied to high volume copiers and printers may be utilized to increase the available number of electrophotographic operations, such means result in added mechanical complexity and cost. As an example, a photoconductive belt may be mounted inside a purely mechanical drum whose external surface is covered by the belt, which, as it is unwound exposes a fresh photoconductive surface. See Schein, supra.
Thus it can be seen that a method of three dimensional freeform fabrication which allows for a larger number of operations without significant degradation of the latent imaging member will result in fewer requirements for the object building powders, mechanical simplification, and lower costs.